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What is an asymptote and why doesn't parabola have one?

  1. Oct 1, 2007 #1
    The thing I have noticed is that parabola's angle from the x axis keeps increasing...meaning the object is decreasing from that direction...if that is the case, then how does it go on in that direction forever even though, its speed going in that direction keeps decreasing?

    At what rate something is decreasing if it has a parabola?(Plz don't explain it in formula, I know that explanation too...I just want more conceptual explanation) I know when x is in the denominator, that when there are 2 asymptotes.
  2. jcsd
  3. Oct 1, 2007 #2
    A function f(x) has an asymptote if it "looks like" a straight line when x tends to [itex]\pm\infty[/itex]. More precisely, if the distance [itex]|f(x)-\text{the line}|[/itex] tends to 0.

    A parabola doesn't have one. Suppose there was one (draw an arbitrary line in the same graph). Then, at some point, the parabola will rush past that line and keep going away from it, so it wasn't an asymptote.
  4. Oct 2, 2007 #3
    Thx for that approach but I guess I wasn't really looking for a counter-example...more like the real WHY? Which is based w/o math...like a drawing...if someone drew an infinite parabola, how can it approach infinity toward the x-axis...b/c the rate at which it is approaching x-axis is decreasing.

    Btw, this brings the question, at what rate is something deaccelerating if a graph has an asymptote? For instance: y=1/x
  5. Oct 3, 2007 #4


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    The words "accelerating" and "decelerating" have nothing to do with parabolas. If you can phrase the question purely in terms of mathematics rather than physics we might be able to understand it better.

    The answer to your first question is really in what you said: "parabola's angle from the x axis keeps increasing". Since it does not approach any one angle, it cannot have an asymptote. The graph of y= x2 "can approach infinity toward the x-axis" because x2 is defined for all x, not matter how large!
  6. Oct 3, 2007 #5
    But it IS approaching an angle! 180 degrees from the x-axis.
  7. Oct 3, 2007 #6
    I wouldn't look at it in terms of increasing angles. I think that will lead to trying to visually evaluate your question. Think of more like, for each value of X, Y gets much bigger than X. An asymptote is a place where a function is undefined. X^2 is never undefined so it can not have an asymptote. In a nut shell there is not a value of X that can't be squared, so if X can always be squared then any value of X can be used, therefore X can approach infinity.
  8. Oct 4, 2007 #7
    Well yeah, but as I mention before...I was looking for NONMATHEMATICAL example. It is similar to like this...the doctor tells me, I am dead...but I say...I clearly don't feel dead...he says...well...the machines indicate you are dead so you are dead.

    I mean I see that MATH indicate it will go to infinity. But when I look it from my eyes, that is just not what I see. And I just wish there was a more satisfying way to understand this.
  9. Oct 4, 2007 #8
    You can't define something approaching infinity by "looking at it". What you think is common sense may not be quite how it works. Also, I did not give you a mathematical example. There were no calulations in what I posted. It goes to infinity is the answer your looking for. If it doesn't stop how can it approach an asymptote?
  10. Oct 6, 2007 #9
    I see the problem. It is right to say that the angle of the section you are looking at will keep increasing with respect to the x-axis. If I understand this correctly, the angle will approach 90 degrees. 180 degrees would mean it is parallel to the x-axis if you are measuring with respect to it.

    You can look at it from the angles all you want and that is perfectly correct. However, while every function that does have an asymptote does approach an angle just like the parabola, that is not sufficient grounds to conclude that IF a function approaches an angle THEN it is asymptotic. It's true that asymptotic functions do that. But it is not true that all functions that do that are asymptotic. Tricky, tricky.

    The way I like to break it down is this. There is a group of functions that approach a given angle. Some of them are asymptotic, but not all of them. Why? Because an asymptote means the functions can never quite get to some value or other that it gets real close to, and as a result has this angle business. But for a parabola, the function can take in any x value and output any y value. (Not every x is paired with every y.) Actually, it only outputs every y value equal or greater than the bottom of the parabola, of equal to or less than the top if it is pointing down.

    The point is: the function must lack the ability to reach a certain VALUE and approach that value arbitrarily closely. I think the difficulty is that, although the parabola gets steeper and steeper, it still keeps going. You just have to back up and "look at" more.

    What might break your spell is this. The parabola can take in ANY x value as its input. The height is the x value squared. Where is the y value? Well, it is straight above the x! So if we can do this for any x value, then we might as well be plotting y = x because that does the same thing. It takes in a "width" and outputs a "height" directly above it. Alas, the x in y = x is not squared so the heights are not as high, but that does not matter. That has to do with the shape not the relationship I am talking about here (how y's get mapped from the x).

    If there is a shift left or right, it goes over that many units so its no problem. Just shift your axes and you are back to the original. Same for up or down.

    To repeat, anywhere on the x axis you choose, there is a y some distance above the x. The thing is not limited by width and certainly not by height, although it can't go above/below the vertex or whatever it's called.
    Last edited: Oct 6, 2007
  11. Oct 9, 2007 #10
    OMG, someone finally understood my question! lol, but still doesn't solve the problem...the thing is...I like to look at it as if it is similar to that paradox....here it is

    Paradox: Lets say you want to travel 1 meter, what you do is that you travel half the distance: .5 meter, then travel half the distance of what you have already traveled:.5 and keep repeating the process infinitely so you keep getting closer, but never get there.

    Based on this paradox...I believe that if it is keep approaching 90 degrees angle....even though it is getting closer to infinity, it can never achieve it conceptually..get it?
  12. Oct 9, 2007 #11
    infinity has to do with height, not angle. y = x goes to infinity in either direction, but it is a clear 45 degrees with respect to the x-axis no matter where you are.
  13. Oct 9, 2007 #12


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    That's Zeno's paradox of the arrow, yes? The resolution is that you *can* travel an infinite number of steps infinite time, provided the total distance is finite (and sufficiently small).

    Because the parabola has an increasing slope (= angle from the x-axis) it cannot have an asymptote other than a vertical asymptote. If the parabola had a vertical asymptote then there would be some point where you could get infinitely large values as you became arbitrarily close to that point, but there is no such point on the parabola. Take y=x^2 as an example; for every x the limit as x approaches k of y is just k^2, which is finite; it's not unbounded in any finite interval.
  14. Oct 11, 2007 #13
    See thats the point...all the explanations that people have been giving me have to do w/ mathematical calculations...why can't someone just teach me the concept WHY it does approach infinity?
  15. Oct 11, 2007 #14
    Ok. no sweat. Its because x^2 is defined for all x. x^2 gets bigger and bigger, faster and faster, but there is no x where it gets infinitely large. It keeps getting steeper and steeper. Its slope approaches verticality. But thats not enough to have an asymptote. You have to approach vertically *fast enough*. x^2 does not. it takes infinitely many x s before x^2 becomes vertical.

    If you forgive the mathematical example, consider the series SIGMA(1/n) . Each step is smaller than the last. The size of the terms goes to zero, and yet their sum is unbounded. What does this have to do with the parabola? Well, if you look at the distance it takes for the slope of the parabola to double, that distance gets smaller and smaller as the x gets larger. But it still takes infinitely large x until the slope goes to infinity.

    Other functions, like 1/x, do have asymptotes because their slope increases much more quickly than x^2
  16. Oct 11, 2007 #15


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    An asymptote is a straight line associated with a curve such that as a point moves along an infinite branch of the curve the distance from the point to the line approaches zero and the slope of the curve at the point approaches the slope of the line.

    The word's origin is Greek and means "not intersecting". It's used in mathematics to merely describe an imaginary line that a curve cannot cross.

    Is that conceptual enough?
  17. Oct 11, 2007 #16
    LOL....did you even read my question? I am not asking what an asymptote is...I am asking...why doesn't parabola has a verticle one.

    btw,(this is a mathematical question :-) at what rate is something decreasing if it has an asymptote?
  18. Oct 11, 2007 #17


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    What is an asymptote and why doesn't parabola have one?

    Is this not the title of this thread??
  19. Oct 11, 2007 #18


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    So first you don't like the mathematical answers, but then you admit its a mathematical question??
  20. Oct 11, 2007 #19


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    A parabola is a continuous function. That means it doesn't have any discontinuities. An asymptote creates a discontinuity in the would be path of a parabola.

    An asymptote is a straight line; its rate of change is zero.
  21. Oct 11, 2007 #20


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    Consider the graph of y = 10 / (x^2 + 1).

    Is the line y=0 an asymptote?
    Is the line y=1 an asymptote?
    Is the line y=-1 an asymptote?

    (The answers are not all the same)

    Incidentally, projectively, asymptotes are simply tangent lines: they are tangent at infinity. A parabola has only one point at infinity, and the tangent line there is the line at infinity.
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